A PM motor refers to a direct current (DC) brushless permanent magnet synchronous motor, also known as an electronically commutated motor (ECM). Based on the application in which the motor is used, the control mode for the motor may be different. Common control modes include constant speed control, constant torque control, and constant airflow control. Constant airflow control is a common control mode for motors in HVAC systems.
In an indoor ventilation duct for a household air conditioner, the static pressure often changes with the passage of time, for example, as a result of pipe coating or filter clogging. Static pressure is often higher than the standard static pressure for the nominal system tested in the manufacturer's laboratory because pipe installations are different. Constant airflow control may provide constant airflow to users in these cases to maintain a comfortable ventilation, cooling, or heating effect under a wide range of static pressure conditions, and to make the system achieve high efficiency and energy saving.
A common control method for constant airflow requires a directly-installed air flow meter that not only increases the cost, but also increases the potential for failure because of the failure of the air flow meter. Current air-conditioning manufacturers often use a control method for constant airflow without the air flow meter. Some control methods need to monitor the variation of static pressure to adjust speed, and some calculations involve logarithmic computations or high-order polynomials, which require a microcontroller unit (MCU) of a motor controller to have powerful computational capability, which further increases the cost.
U.S. Pat. No. 4,806,833 discloses a method to change the motor speed based on the torque and the external static pressure to obtain constant air volume. To calculate the external static pressure change based on the variation of rotating speed induced by the motor's own tachometer, an airflow calculation may be obtained as a function torque and rotational speed. U.S. Pat. No. 5,736,823 also discloses a control method for constant air volume that is also a function of the torque and rotational speed.
With respect to the above-described control methods for constant airflow using torque as the key control variable, there are the following technical problems. Torque is a mechanical variable that is difficult to measure and calculate, and that will affect the precision of the control. Torque control is often used in the application of dynamic control, but constant airflow control is typically a steady state control, therefore not suited for torque control. Torque control is more complicated in motor control. Most likely, vector control is required to achieve a good torque control, but the precision of the system torque control is constrained by the cost of the system hardware and software. Torque is not a characteristic parameter by which a consumer can understand the operation of the system or for government regulation. In other words, torque is a mechanical variable that is difficult to directly measure, and using torque as a control variable to realize constant airflow control leads to complex calculations and high cost. In addition, torque is not a good characteristic by which consumers can understand system operation, nor is it a characteristic by which compliance with government regulations can be verified.